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NB 6 speed swap retaining 4.30 diff.

Old 01-09-2018, 10:05 PM
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Default NB 6 speed swap retaining 4.30 diff.

I did a lot of searching, pretty much everywhere on this topic, and found little about it. So, with a broken 5th gear, and plans for boost in the future, I decided to use a 2000 NB 6 speed I found used up in El Monte from a real cool dude that has a lot of parts available at great prices. Mine needed to be cleaned, I changed the seals front and rear, and then took off the tail shaft housing to do some inspection before putting it in. Had some trouble with the shifter, but that was my own fault - nothing big, just some identification of shifters that fit and don't fit. I educated myself on the differences between the NB1 and NB2 6 speeds, ordered the parts I needed, and got started.

First off, I swapped out the old clutch and flywheel for a new one, and for that I used a SuperMiata kit with a 4 puck sprung ceramic disc, and lightened flywheel. I did some comparisons between the weights of the complete clutch/flywheels assemblies, then posted that info elsewhere in another thread on that topic. Difference was pretty significant, and you can feel it - in a good way, but the clutch takes a little getting used to, and you don't want to cruise around in a lot of slow traffic if you can avoid it. Other than that, I'm happy with it, and stay away from traffic. Shots of the install...


Had a lot of help from my brother. Just having someone to work top and bottom made it a lot easier.


Flywheel torqued to spec and cleaned so there were no oil or lubricant contaminants.

Included alignment tool worked just as good as any other alignment tool I've used.

Pressure plate installed. Taking photos under the car wasn't easy. Basically, 2 old guys doing young guy stuff.
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Old 01-09-2018, 10:14 PM
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Besides the clutch I also rebuilt the shifter with all factory parts other than the addition of a 5X brass bushing, and a 1320 Racing weighted shift ****, and I gotta say, I love the way it feels, and the **** fits just right, and smooths out shifts so they are not too unlike the 5 speed it replaced. So far the only shift I can complain about are 4-5, but I'm getting used to it. **** shots follow:



I purchased this 1320 Racing weighted **** from Amazon for something like $50 and its worth every penny. If you don't know what 1320 is, Google it. My **** is hot when its hot, cold when its cold, but shifts right every time. Check the "0" in the logo. Just the right touch.

Car is a little dirty. I drive with the top down most of the time.

Promise, I'm going to clean this up as soon as it stops raining here in So Cal. Taken in my garage tonight.

So, 5X kit and 1320 shift ****, can't say enough good about them, would definitely purchase both again without a second thought. OH, don't waste your money on shifter alignment spacers. I used some 1/2 inch PVC pipe cut and cut again until the shifter logo was straight and the shifter was tight against the boot. Spacer goes under boot and fits against taper of shifter, and sandwiches boot between the two. Would have posted a pic, but forgot. If you really want to see one, I can post later, but will have to remove **** to do it.

Last edited by Long_av8r; 01-09-2018 at 10:45 PM. Reason: Fix top shifter pic.
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Old 01-09-2018, 10:25 PM
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Now, how does it perform with the stock 4.30 diff? Awesome. If you want to drive at 2500 rpm. this is not for you. If you don't mind being above 4k most of the time and have a non-boosted car, the 4.30 works. I'll post up some speeds/gear/rpm specs later, but first isn't too low, and 6th for hwy cruise at 80 mph is right around 4250, so just fine by me. Don't know what top end is, but truth be told, I didn't know what it was on my 5 speed either. I know this, its fast enough for a ticket.

Bottom line is this; if you want to gradually modify your car and spread funds out, this is a good way to get a bump on the bottom end without sacrificing much on top. In traffic, 2nd gear pretty much does it for stop and go, and 3rd carries you in 45 mph steady traffic without having to shift up or down. I find this 6 speed with the 4.30 rear to be far better than my 5 speed with the same ratio. I may install a 4.10 before I boost just to see where it goes when I do boost. If I believe everything I read on the internet, I'll eventually have to upgrade to an LSD with a 3.93 or 3.63, but I don't believe everything I read on the internet, so I experiment. The other day I experimented with a lot of throttle mid corner in 2nd, and the car drifted nicely and recovered like a Cessna 152 in full spin.

So that's it. I'll be here to answer questions all week..
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Old 01-09-2018, 10:43 PM
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One last note on the clutch... Wife hates it, but she drives a Yukon all the time so not used to a clutch anymore - hasn't had one since the new VR4 I got her in the mid 90's and that's a long time ago - but she hasn't forgotten, just needs more time with it. However... you won't be able to teach your teenage daughter how to use a manual transmission in it. And that sucks because I have two of those - both of which want their own Miata. Now, that's raising them right. My oldest giggles like crazy every time I "slip" it. Haha, she says slip, I say slide, maybe its a little of both.
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Old 01-10-2018, 01:20 AM
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Wheeeeew those are short gears.
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Old 01-10-2018, 05:28 AM
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I always thought a 6sp/4.30 setup would be a lot of fun in an n/a car. I'm surprised it's not done more often.
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Old 01-10-2018, 07:08 AM
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For an atmo car, the 6-speed/4.3 rocks on the track - well some tracks, depending on whether your rev limit is 7, 8, or 9k rpm, and how much time you lose feathering it on the redline compared to gains elsewhere if there are long straights. I tried the 4.7 but it was a step too far for me/my tracks, but on a short track worked well with an 8k rev limit. Work being the operative word, the shifts come fast and frequent.

For a turbo car, not so sure. I am about to track my SE (MSM), which came fitted here with the 3.6, so I will soon find out, but on the road I am getting that same feeling as I had with the 4.7 - the shifts are coming quick when the welly is applied, and I expect this might be more extreme when tracked. OTOH it might be something moderated with more seat time (and a higher rev limit than the stock ECU allows). I think the 4.1 would be far too short for a big hp turbo car though, but happy to hear from someone who has found it different.

Having played around with various combinations of 5- and 6-speed boxes running 4.1, 4.3 and 4.7 final drives, I have concluded that it is all about what the characteristics of your tracks are, and the trade-off between acceleration and top speed - unless you (unlike me) are prepared to swap out diff centres between events.
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Old 01-10-2018, 09:58 AM
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Originally Posted by pdexta View Post
I always thought a 6sp/4.30 setup would be a lot of fun in an n/a car. I'm surprised it's not done more often.
I have 2 cars with it. It's rad.
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Old 01-10-2018, 01:34 PM
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I ran a 6 speed with a 4.30 in the original K24 swapped car for a while. It was great on some tracks but not so great on others. 220whp meant you'd rip through the gears pretty fast and see 5th gear all the time, but others have shown that a 6 speed and 4.30 is the fastest option for an N/A BP with less power. I ended up swapping to a 4.10 and it was perfect. I'm with Gee Emm, I don't have the motivation to change rear ends between events, so I like to pick one and stick with it.
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Old 01-10-2018, 10:42 PM
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Just went out to see what my speed is at 4250, and Im right at 90mph. Honestly less of a difference than I thought.
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Old 01-11-2018, 12:46 PM
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Originally Posted by thumpetto007 View Post
Just went out to see what my speed is at 4250, and Im right at 90mph. Honestly less of a difference than I thought.
You can just play with a gearing calculator for a while!
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Old 01-11-2018, 05:59 PM
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But that doesnt put the wind beneath my sails.
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Old 01-11-2018, 06:01 PM
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Nor does it let us use our inaccurate tachs to get mediocre data.
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Old 01-11-2018, 06:14 PM
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It's not brain surgery.

Just look at the % difference 3.6/4.1/4.3/4.7 and that will tell you how much difference it will make to cruising revs (if road car), and speed in gears/top speed.
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Old 01-11-2018, 06:15 PM
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double post

Last edited by Gee Emm; 01-11-2018 at 06:29 PM.
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Old 01-12-2018, 10:20 AM
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I'm working with a friend to translate my gearing comparison spreadsheet to a web based setup. It'll displays the speeds graphically for each gear so you can see the whole picture and not just top speeds in X gear.
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Old 01-12-2018, 10:22 AM
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Nice.

MFactory had a really good one that did that, but it hasn't been back up since they re-did their website. It is sorely missed.
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Old 01-12-2018, 11:12 AM
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